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Sizing Neutrals for Switching Power Supply Loads -Guy Macon

Discussion in 'Electronic Design' started by Guy Macon, Dec 5, 2007.

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  1. Guy Macon

    Guy Macon Guest

    You are incorrect. The above is only true in the special case where
    the loads have current waveforms that cancel, such as resistive loads,
    matched inductive loads, or power-factor-corrected power supplies as
    loads (often found in servers). The above claim is *NOT TRUE* for
    cheap switching power supplies sharing a neutral. In that case each
    phase has a huge current spike at the peak of the voltage waveform
    (think capacitor being kept charged through a bridge rectifier).

    I strongly recommend that you read section 7 ("Why do 3rd harmonic
    currents overload neutral conductors") of the following FAQ:
    [ http://www.mirusinternational.com/pages/faq.html ]

    There are harmonic standards (USA IEEE Standard 519-1992, EU
    Standard EN61000-3-2) that attempt to put the burden of keeping
    these harmonic currents at a safe level on equipment manufacturers,
    but the person who buys a couple of hundred cheap personal computers
    and installs them in an old buildings to replace electric typewriters
    often doesn't know anything about this and ends up overheating the
    neutral.

    Also see:

    [ http://www.springer.com/west/home?SGWID=4-102-45-131253-0 ]
    [ http://www.dataforth.com/catalog/pdf/an108.pdf ]
    [ http://www.smartdev.com/pdf/ac_power.pdf ]
    [ http://www.mirusinternational.com/downloads/hmt_faq06.pdf ]
    [ http://www.mirusinternational.com/downloads/hmt_faq07.pdf ]
    [ http://www.mirusinternational.com/downloads/hmt_faq08.pdf ]
    [ http://www.mirusinternational.com/downloads/hmt_faq09.pdf ]
    [ http://www.mirusinternational.com/downloads/hmt_faq10.pdf ]
    [ http://cms-emc.web.cern.ch/cms-emc/pdffiles/PhDfiles/grounding.pdf ]
    [ http://www.thecircuitdetective.com/twocircuit.htm ]







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  2. bud--

    bud-- Guest


    I would say the general case is neutral balance with harmonics being the
    special case.

    I considered adding a comment on harmonics. The thread was residential
    and harmonics are not a major problem. If one fully qualifies all
    possibilities in answers they can become unreadable.

    Harmonics certainly can be a problem, but probably not even on most
    commercial/industrial 15/20A circuits. My impression is they are more of
    a problem on 3 phase because of triplen adding.
     
  3. Guest

    That is true but in commercial applications and even some large multi
    family dwellings like high rise condos, the supply is 208 3 phase. The
    recomendation is that you upsize the neutral 2 wire sizes if there is
    a significant harmonic load.
     
  4. Guy Macon

    Guy Macon Guest

    ....or if you are unsure as to whether there will be a significant
    harmonic load some time in the future. It is a Good Thing if the
    fellow who decides to put a really big arc welder in his garage,
    turns his house into a marijuana farm, or to run racks and racks
    full of computers (Google started by renting space in a residential
    garage, IIRC) manages to do so without setting the wiring on fire.
     
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